1. Field of the Invention
The present invention relates to a method of and a power plant for efficient oxyfuel combustion, and to a method of modifying a process of generating power by combusting carbonaceous fuel from combusting the fuel with air to combusting the fuel with substantially pure oxygen. More particularly, the present invention relates to oxyfuel combustion including carbon dioxide removal, wherein the costs or losses of produced power due to the carbon dioxide removal are minimized.
2. Description of the Related Art
Oxyfuel combustion is one of the methods suggested for removing CO2 from the combustion gases of a power generating boiler, such as a pulverized coal (PC) boiler or circulating fluidized bed (CFB) boiler. Oxyfuel combustion is based on combusting carbonaceous fuel with substantially pure oxygen, typically of about 95% purity, so as to have carbon dioxide and water as the main components of the exhaust gas discharged from the boiler. Thereby, the carbon dioxide can be captured relatively easily, without having to separate it from a gas stream having nitrogen as its main component, as when combusting the fuel with air.
Oxyfuel combustion is more complicated than conventional combustion because of the need of an oxygen supply, typically, a cryogenic air separation unit (ASU), where air is cooled and compressed so that oxygen can be separated from other components of air, mainly, nitrogen. CO2 can be separated from the exhaust gas, for example, by cooling it to a relatively low temperature and by compressing it to a high pressure, typically, greater than 110 bar. Both the production of oxygen and the compression/purification of carbon dioxide increase the total production costs of the power generation process, for example, by decreasing the net power produced in the process.
The steam cycle of a utility boiler comprises a number of heating stages, wherein low temperature feedwater obtained from a condenser is converted to high temperature steam, which will be conducted to a high pressure steam turbine. The process of heating the feedwater comprises the stages of preheating, evaporating and superheating. Normally, the feedwater preheating consists of heating with steam, which is extracted from the steam turbines, and final preheating in one or more economizers arranged in the downstream portion of the exhaust gas channel. An example of a conventional steam cycle of a utility boiler is shown is U.S. Pat. No. 4,430,962.
Evaporative heat exchange surfaces are usually arranged as waterwalls of the furnace, and superheating surfaces, and possible reheating surfaces, are most often arranged in the upstream portion of the exhaust gas channel. In order to increase thermal efficiency of the boiler, in the downstream portion of the exhaust gas channel is usually arranged an air heater downstream of the economizer, wherein the exhaust gas will be cooled to a final temperature, which is typically from 150° C. to 120° C. An example of a utility boiler with an air heater is shown in U.S. Pat. No. 4,205,630.
U.S. Pat. No. 6,202,574 suggests a combustion unit for firing fossil fuel with substantially pure oxygen to produce exhaust gas having carbon dioxide and water as its two largest constituents. A portion of the exhaust gas is recycled to the combustion unit and the rest of the exhaust gas is compressed and stripped to produce carbon dioxide in a liquid phase. The recycled exhaust gas and the substantially pure oxygen stream are preheated by the exhaust gas in respective gas-gas heat exchangers.
U.S. Pat. No. 6,935,251 suggests a method of combusting fuel with an oxidant stream comprising an oxygen enriched gas stream, having an oxygen content from 21% to 100%, mixed with flue gas, so as to generate flue gas having a mass flow rate which is smaller than the mass flow rate of flue gas generated by using air as the oxidant stream, whereby the size of the flue gas pollution control system can be minimized.
U.S. Pat. Nos. 5,344,627 and 6,883,327 each describes a conventional steam-generating process, where CO2 is removed from the exhaust gas through absorption by a solvent, wherein a portion of outlet steam from high pressure or intermediate pressure steam turbines is expanded in auxiliary turbines to drive a carbon dioxide compressor and auxiliary machinery. These processes can be used to minimize, to some extent, the costs or the decreasing of the net power produced in the process.